Accurate control of temperature in an injection molding apparatus is fundamental to maintaining control of throughput rate and product quality in an injection molding process. Heaters are typically provided to heat the melt flowing through the manifold and nozzles and cooling channels are provided to cool the melt in the mold cavities. During injection, the melt must be maintained within a temperature range dictated by the melt material. Once the melt has been injected into the mold cavities, the melt is cooled at a predetermined rate to produce molded parts. The predetermined cooling rate is calculated based at least in part on the temperature of the melt as it enters the mold cavities.
In a multi-cavity injection molding apparatus, the temperature of the melt entering the mold cavities often varies from one mold cavity to the next. As such, the optimum cooling time for the plastic in each mold cavity may be slightly different. For injection molding applications in which semicrystalline resins are used, this temperature variation often results in the production of molded articles that are of insufficient quality.
A common application of semicrystalline resins is in the production of polyethylene terephthalate (PET) preforms. In order to produce high quality preforms, the semicrystalline resin must be cooled in the mold cavity for a sufficient period of time to allow the preform to solidify before being ejected, while avoiding the formation of crystalline portions. Crystalline portions typically form in the bottom portion of the preform adjacent the mold gate. The crystalline portions cause the preform to become brittle so that it may crack when it is blow molded.
There have been many attempts to optimize the cooling of PET preforms in order to produce high quality molded products efficiently. For example, U.S. Pat. No. 6,171,541 entitled “Preform Post-Mold Cooling Method and Apparatus” issued to Husky Injection Molding Systems Ltd. on Jan. 9, 2001, discloses a rapid injection molding process where the molded articles are ejected from the mold before the cooling step is complete.
U.S. Pat. No. 6,276,922 entitled “Core Fluid Velocity Inducer” issued to Husky Injection Molding Systems Ltd. on Aug. 21, 2001, discloses an inducer located at the outlet of a cooling supply tube for improving the circulation of the cooling supply throughout the core.
U.S. Pat. No. 6,176,700 entitled “Injection Molding Cooled Cavity Insert” issued to Jobst Gellert on Jan. 23, 2001, discloses an injection molding apparatus having a cavity insert with a cooling fluid flow channel extending between integral inner and outer portions thereof. The cavity insert attempts to improve the cooling process for molded articles. The nozzle includes a thermocouple that measures the temperature of the molten material as it leaves the nozzle.
Despite all of the attempts to improve the cooling process for molded articles, the method of measuring the temperature of the molten material in the mold cavity has not improved. It is desirable to obtain additional temperature measurements at the outlet of the nozzle because large temperature variations may occur in this area. It is therefore an object of the present invention to provide a method and apparatus for measuring the temperature of the molten material in the mold cavity.